The communications and radar industries have had considerable interest in microwave amplifier power combiners featuring non-overmoded compactness, thermal robustness, high combining efficiency, and the ability to perform over a large bandwidth.
One power combining method uses a corporate ‘tree’ structure (see FIG. 1; see also Kenneth J. Russell, “Microwave power combining techniques,” IEEE Trans. on Microwave Theory and Techniques, May 1979, pp. 472-478). For example, FIG. 1 is a block diagram of an 8-input, three-stage conventional corporate ‘tree’ structure that comprises eight isolator-protected source modules 1, 2, 3, 4, 5, 6, 7, and 8 of equal frequency, magnitude, and relative phase delivering combined power to a load 16 through seven 2-input combiner subunits 9, 10, 11, 12, 13, 14 and 15. Each of the 2-input combiner subunits 9-15 may be a three-port structure (two inputs, one output) for which not all ports can be impedance-matched. Alternatively, each of the 2-input combiner subunits 9-15 may have a source isolation port in addition to its output port, thus making it a four-port structure with all ports impedance-matched. The three combining stages include stage ‘A’ formed of the combiner subunit 15, stage ‘B’ formed of the combiner subunits 13 and 14, and stage ‘C’ formed of the combiner subunits 9-12. Transmission lines 17, 18, 19, 20, 21, and 22 separate each stage. Typically, each transmission line 17-22 has a uniform impedance, impedance-matched to its mating port on each end, and has a length compatible with convenient separation of the 2-input combiner subunits 9-15. The large number of separate components for this prior art approach, and the physical space used for the overall structure, is often problematic, especially for low-frequency applications.
Another restriction is its useful bandwidth, which is limited to that of the individual combiner subunits. This bandwidth is further compromised due to adverse summing of the individual stage vector reflection coefficients within the corporate combiner structure.
An additional disadvantage of this prior art approach is that the combining efficiency of the corporate structure is compromised. This is due to the large number of separate components which contribute RF losses and also due to stage-to-stage reflection coefficient scattering which exacerbates the overall combiner loss.
The corporate ‘tree’ approach typically uses 2-input combiner subunits. This limits application to 2N input sources, where N is the number of combining stages. Therefore it is not possible to power combine, say, twelve input sources using the prior art corporate structure approach.